If you want to back up the OneDrive folder of each account on the computer, unless they've been deliberately moved elsewhere you can find them under C:\Users<accountname>\OneDrive. Create a folder for each user on the secondary drive and then copy their respective OneDrive folders there under a folder indicating which is which, like D:\OneDriveBackup\User1, ...User2... etc. There is also a free tool that Microsoft used to offer called SyncToy that would copy one folder to another (you can still find it with a web search) or you can use a built-in command line tool called robocopy and you can create a batch file that will use robocopy to copy the OneDrive folders all at once. This is fairly easy to create and I've used robocopy for duplicating folders between drives.
As for paid backup tools, there are many that are free, like the AOMEI Backupper tool I mentioned. There's also Macrium Reflect Free and others as well.